1. Field of the Invention
Generally, the present disclosure relates to sophisticated integrated circuits including transistor elements comprising gate structures formed on the basis of a high-k gate dielectric material and a metal-containing electrode material.
2. Description of the Related Art
The fabrication of advanced integrated circuits, such as CPUs, storage devices, ASICs (application specific integrated circuits) and the like, requires the formation of a large number of circuit elements on a given chip area according to a specified circuit layout, wherein field effect transistors represent one important type of circuit element that substantially determines performance of the integrated circuits. Generally, a plurality of process technologies are currently practiced, wherein, for many types of complex circuitry including field effect transistors, CMOS technology is currently one of the most promising approaches due to the superior characteristics in view of operating speed and/or power consumption and/or cost efficiency. During the fabrication of complex integrated circuits using, for instance, CMOS technology, millions of transistors, i.e., N-channel transistors and P-channel transistors, are formed on a substrate including a crystalline semiconductor layer. A field effect transistor, irrespective of whether an N-channel transistor or a P-channel transistor is considered, comprises so-called PN junctions that are formed by an interface of highly doped regions, referred to as drain and source regions, with a slightly doped region, such as a channel region, disposed adjacent to the highly doped regions. In a field effect transistor, the conductivity of the channel region, i.e., the drive current capability of the conductive channel, is controlled by a gate electrode formed adjacent to the channel region and separated therefrom by a thin insulating layer. The conductivity of the channel region, upon formation of a conductive channel due to the application of an appropriate control voltage to the gate electrode, depends on, among other things, the dopant concentration, the mobility of the charge carriers and, for a given extension of the channel region in the transistor width direction, the distance between the source and drain regions, which is also referred to as channel length. Hence, the conductivity of the channel region substantially affects the performance of MOS transistors. Thus, the scaling of the channel length, and associated therewith the reduction of channel resistivity, is a dominant design criterion for accomplishing an increase in the operating speed of the integrated circuits.
Presently, the vast majority of integrated circuits are based on silicon due to substantially unlimited availability, the well-understood characteristics of silicon and related materials and processes and the experience gathered during the last 50 years. Therefore, silicon will likely remain the material of choice in the near future for circuits designed for mass products. One reason for the importance of silicon in fabricating semiconductor devices has been the superior characteristics of a silicon/silicon dioxide interface that allows reliable electrical insulation of different regions from each other. The silicon/silicon dioxide interface is stable at high temperatures and, thus, allows performing subsequent high temperature processes, as are required, for example, for anneal cycles to activate dopants and to cure crystal damage without sacrificing the electrical characteristics of the interface.
For the reasons pointed out above, in field effect transistors, silicon dioxide is preferably used as a base material of the gate insulation layer that separates the gate electrode, frequently comprised of polysilicon or metal-containing materials, from the silicon channel region. In steadily improving device performance of field effect transistors, the length of the channel region has been continuously reduced to improve switching speed and drive current capability. Since the transistor performance in terms of switching speed and drive current is controlled by the voltage supplied to the gate electrode to invert the surface of the channel region to a sufficiently high charge density for providing the desired drive current for a given supply voltage, a certain degree of capacitive coupling, provided by the capacitor formed by the gate electrode, the channel region and the silicon dioxide disposed therebetween, has to be ensured. It turns out that decreasing the channel length requires an increased capacitive coupling to avoid the so-called short channel behavior during transistor operation. The short channel behavior may lead to an increased leakage current and to a pronounced dependence of the threshold voltage on the channel dimension. Aggressively scaled transistor devices with a relatively low supply voltage and thus reduced threshold voltage may suffer from an exponential increase of the leakage current, while also requiring enhanced capacitive coupling of the gate electrode to the channel region. Thus, the thickness of the silicon dioxide layer has to be correspondingly decreased to provide the required capacitance between the gate and the channel region. For example, a channel length of approximately 0.08 μm may require a gate dielectric made of silicon dioxide as thin as approximately 1.2 nm. Although generally usage of high speed transistor elements having an extremely short channel may be substantially restricted to high speed signal paths, whereas transistor elements with a longer channel may be used for less critical signal paths, such as storage transistor elements, the relatively high leakage current caused by direct tunneling of charge carriers through an ultra-thin silicon dioxide gate insulation layer may reach values for an oxide thickness in the range of 1-2 nm that may not be compatible with thermal design power requirements for performance driven circuits.
Therefore, replacing silicon dioxide-based dielectrics as the material for gate insulation layers has been considered, particularly for extremely thin silicon dioxide-based gate layers. Possible alternative materials include materials that exhibit a significantly higher permittivity so that a physically greater thickness of a correspondingly formed gate insulation layer provides a capacitive coupling that would be obtained by an extremely thin silicon dioxide layer.
Additionally, transistor performance may be increased by providing an appropriate conductive material for the gate electrode so as to replace the usually used polysilicon material, since polysilicon may suffer from charge carrier depletion at the vicinity of the interface to the gate dielectric, thereby reducing the effective capacitance between the channel region and the gate electrode. Thus, a gate stack has been suggested in which a high-k dielectric material provides enhanced capacitance based on the same thickness as a silicon dioxide-based layer, while additionally maintaining leakage currents at an acceptable level. On the other hand, the non-polysilicon material, such as titanium nitride and the like, in combination with other metals, may be formed so as to connect to the gate dielectric, thereby substantially avoiding the presence of a depletion zone. Since the threshold voltage of the transistors, which represents the voltage at which a conductive channel forms in the channel region, is significantly determined by the work function of the metal-containing gate material, an appropriate adjustment of the effective work function with respect to the conductivity type of the transistor under consideration has to be guaranteed.
Providing different metal species for adjusting the work function of the gate electrode structures for P-channel transistors and N-channel transistors at an early manufacturing stage may, however, be associated with a plurality of difficulties, which may stem from the fact that a complex patterning sequence may be required during the formation of the sophisticated high-k metal gate stack, which may result in a significant variability of the resulting work function and thus threshold of the completed transistor structures.
For this reason, in some approaches, the initial gate electrode stack may be provided with a high degree of compatibility with conventional polysilicon-based process strategies and the actual electrode metal, possibly in combination with a high-k dielectric material, and the final adjustment of the work function of the transistors may be accomplished in a very advanced manufacturing stage, i.e., after completing the basic transistor structure. In a corresponding replacement gate approach, a standard polysilicon or amorphous silicon material may be patterned on the basis of well-established advanced lithography and etch techniques. After patterning the gate electrode structure, conventional and well-established process techniques for forming the drain and source regions having the desired complex dopant profile are typically performed. After any high temperature processes, the further processing may be continued by the deposition of an interlayer dielectric material, such as silicon nitride in combination with silicon dioxide and the like.
In this manufacturing stage, a top surface of the gate electrode structures embedded in the interlayer dielectric material has to be exposed, which is accomplished by chemical mechanical polishing (CMP). The polysilicon material exposed during the CMP process is then removed and, thereafter, an appropriate masking regime may be applied in order to selectively fill in an appropriate metal for any type of transistors.
Although, in general, this approach may provide advantages in view of reducing process-related non-uniformities in the threshold voltages of the transistors, the complex process sequence for exposing and then removing the placeholder material and providing appropriate work function materials for the different types of transistors may also result in a significant degree of variability of the transistor characteristics, which may thus result in offsetting at least some of the advantages obtained by the common processing of the gate electrode structures until the basic transistor configuration is completed.
For example, an efficient removal of the polysilicon material may have a significant influence on the overall characteristics of the replacement gate, i.e., on the provision of appropriate work function metals for the N-channel transistor and P-channel transistor and the subsequent deposition of the actual metal-containing electrode material.
Typically, a dielectric cap material may be preserved throughout the entire processing of the semiconductor devices, which may be advantageous in terms of maintaining integrity of the gate electrode structure prior to replacing the polysilicon material. It turns out, however, that the exposure of the top surface of the polysilicon material may represent a very critical process phase during the replacement gate approach, in particular when a dielectric cap layer, for instance provided in the form of a silicon nitride material, is present on the polysilicon material, as will be explained in more detail with reference to FIGS. 1a-1b. 
FIG. 1a schematically illustrates a cross-sectional view of a semiconductor device 100 in an advanced manufacturing stage. As illustrated, the device 100 comprises a substrate 101, above which is formed a semiconductor layer 102. The semiconductor layer 102 is typically provided in the form of any appropriate semiconductor material, such as silicon and the like, wherein, if a silicon-on-insulator (SOI) architecture is considered, a buried insulating material (not shown) is typically formed directly below the semiconductor layer 102. In other cases, the semiconductor layer 102 represents a portion of a crystalline semiconductor material of the substrate 101, if a bulk architecture is considered. The semiconductor layer 102 comprises a plurality of active regions, wherein, for convenience, a single active region 102a is illustrated in FIG. 1a. Generally, an active region is to be understood as a semiconductor region in and above which one or more transistors are formed. In the example shown in FIG. 1a, a plurality of transistors 150 are formed in and above the active region 102a and may represent transistors of the same conductivity type in a device region in which a plurality of densely packed transistors are required. The transistors 150 typically comprise gate electrode structures 160, the final configuration of which is still to be established on the basis of a replacement gate approach, as discussed above. The gate electrode structures 160 in this manufacturing stage comprise a dielectric material 161, such as a silicon dioxide material and the like, in combination with a polysilicon material 162, which is to be considered as a placeholder material since at least the material 162 is to be removed in a later manufacturing stage. Moreover, a sidewall spacer structure 164 is typically provided which, for instance, enables a desired lateral and vertical profiling of drain and source regions 151. Furthermore, as discussed above, frequently, a dielectric cap layer or cap layer system 163 is provided, which has been removed during the subsequent processing, while in other cases the cap layer 163 may be omitted. Furthermore, a contact level 120 comprising an interlayer dielectric material 121 is provided so as to laterally enclose the gate electrode structures 160. It should be appreciated that, in the manufacturing stage shown, a top surface of the polysilicon material 162 may be exposed, as is required for the subsequent replacement of the material 162 with at least a metal-containing electrode material.
The semiconductor device 100 as shown in FIG. 1a may be formed on the basis of the following processes. The active region 102a is formed on the basis of well-established process techniques, for instance forming isolation structures (not shown), thereby defining the lateral size, position and shape of any active regions in the semiconductor layer 102. Prior to or after forming the corresponding isolation structures, the basic doping characteristics may be adjusted, for instance by ion implantation in combination with appropriate masking regimes, as is well known in the art. Next, the gate electrode structures 160 are formed by depositing or otherwise forming the dielectric material 161, followed by the deposition of the placeholder material 162, for instance in the form of a polysilicon material. Typically, additional hard mask material and other sacrificial materials are to be provided in order to enable the patterning of the resulting layer stack on the basis of sophisticated lateral dimensions as are typically required in advanced semiconductor devices. For example, a critical dimension of the gate electrode structures 160 may be 50 nm and significantly less. For example, the resulting layer stack may also comprise the dielectric cap material 163, for instance in the form of silicon nitride and the like, which may be used as a hard mask material upon patterning the underlying material layers based on sophisticated lithography and etch techniques. Thereafter, a portion of the drain and source regions 151 may be formed, for instance by ion implantation, selective epitaxial growth techniques and the like, depending on the overall process strategy and the device requirements. It should be appreciated that additional processes may be implemented, for instance in order to incorporate strain-inducing semiconductor alloys (not shown) and the like. Moreover, the spacer structure 164 may be completed followed by further implantation processes, if required, in order to establish the final vertical and lateral dopant profile of the drain and source regions 151. This may also involve the application of anneal processes based on any appropriate process techniques. If required, additional metal silicide regions may be formed in the drain and source regions 151, which may also be accomplished on the basis of the sidewall spacer structure 164, wherein the dielectric cap layer 163, if still in place, may efficiently prevent silicidation of the placeholder material 162. In other cases, the formation of a metal silicide in the drain and source regions 151, or at least in a portion thereof, may be accomplished in a later manufacturing stage.
Next, the contact level 120 comprising the interlayer dielectric material 121 is formed, for instance by depositing the dielectric material 121 so as to attempt to substantially completely fill the spaces between the closely spaced gate electrode structures 160. As discussed above, in sophisticated semiconductor devices, the packing density is significantly increased by reducing the critical dimensions and thus also reducing the distance between neighboring circuit components, wherein, in particular, in densely packed device areas such as memory areas and the like, the resulting high aspect ratio created by the closely spaced gate electrode structures 160 may result in deposition-related irregularities, such as the formation of voids in the material 121 between the closely spaced gate electrode structures 160.
As is well known, particularly silicon dioxide is a well-established interlayer dielectric material which may be deposited on the basis of a plurality of deposition recipes, which may have significant differences with respect to their process results. For example, well-established silicon dioxide deposition recipes are available which provide a substantially flow-like deposition behavior, which results in a reliable filling of openings of even high aspect ratio, such as the spaces between the closely spaced gate electrode structures 160. Other deposition techniques for providing a silicon dioxide-like material with superior gap filling capabilities is the spin coating of a dielectric base material having a low viscous state, wherein subsequently a heat treatment may be performed so as to obtain a silicon dioxide-like material upon initiating a material modification and removing any solvents therefrom. It turns out, however, that the deposition techniques providing the superior gap filling capability result in the generation of a silicon dioxide-like material of inferior density and thus quality with respect to the further processing of the semiconductor device 100. That is, typically any such materials may have a reduced density and/or a significantly reduced etch resistivity with respect to a plurality of efficient wet chemical etch recipes, which are typically used for removing contaminants, etching silicon material and the like. Similarly, these deposition recipes providing the superior gap filling capabilities may also result in a material having a significantly reduced resistivity during well-established polishing processes, which are frequently used for removing an excess portion 121e of the interlayer dielectric material 121 in order to finally expose the placeholder material 162.
On the other hand, process techniques are available in which silicon dioxide-like materials with superior density and thus enhanced etch resistivity and mechanical strength may be provided, however, with a pronounced tendency of producing voids upon filling high aspect ratios, such as the spaces between the densely packed gate electrode structures 160. Since any such voids may result in significant device failures upon further processing of the semiconductor device 100, conventional process strategies rely on superior gap filling capabilities and attempt to cope with inferior etch resistivity and/or increased removal rate during a polishing process.
Thus, the material 121 may be provided in a substantially void-free manner on the basis of spin coating, flow-like CVD deposition techniques and the like. Thereafter, the excess material 121e is removed during a planarization process 103, which comprises one or more polishing steps, wherein, at least in a final phase, the dielectric cap layer 163 is to be removed and subsequently the surface of the polysilicon material 162 is exposed. As discussed above, due to the inferior mechanical characteristics of the material 121, however, the polishing rate may be increased, thereby creating recesses 121r laterally adjacent to the gate electrode structures 160, which may be particularly pronounced when the cap layer 163 has to be removed in a final phase of the planarization process 103, since typically it is extremely difficult to obtain a similar removal rate for silicon nitride and silicon dioxide-based materials.
FIG. 1b schematically illustrates the semiconductor device 100 in a further advanced manufacturing stage. As illustrated, the gate electrode structures 160 are now provided in the form of high-k metal gate electrode structures and comprise a high-k dielectric material 165 in combination with two or more metal-containing electrode materials 166, 167. For example, the material 166 may represent any appropriate metal-containing material for adjusting an appropriate work function of the gate electrode structures 160, while the material 167 may be provided in the form of a highly conductive electrode material such as aluminum, aluminum alloys and the like.
The device 100 as shown in FIG. 1b may be formed on the basis of well-established replacement gate strategies in which at least the placeholder material 162 is removed, for instance by using highly selective wet chemical etch recipes, for instance based on tetra methyl ammonium hydroxide (TMAH) or any other ammonium-based etch chemistries. In other cases, in addition to or alternatively, plasma-based etch recipes may be used. During the removal process, the material 161 (FIG. 1a) may act as an efficient etch stop material, which may have to be removed in a further etch process. Consequently, during this process sequence, the material 121 is exposed to various etch atmospheres, thereby also contributing to surface irregularities due to the inferior etch resistivity of the material 121. Furthermore, upon depositing various gate dielectric materials of the layer 165 and various work function adjusting metal layers 166, which may have to be partially removed in some of the gate electrode structures, additional exposure to reactive etch atmospheres may occur, thereby further damaging the material 121. Finally, the actual fill metal 167 is deposited by any appropriate deposition techniques and excess materials of the previous process sequence are removed, for instance on the basis of CMP and the like. Due to the damaged material 121 and the previously created recesses 121r (FIG. 1a), there is a significant probability that metal residues 167r remain on the material 121, wherein at least some of these residues 167r may cause leakage paths or even short-circuits between neighboring gate electrode structures or between contact elements still to be formed in the material 121 in a later manufacturing stage.
Consequently, although a replacement gate strategy provides a very efficient manufacturing process for providing high-k metal gate electrode structures in a late manufacturing stage, i.e., after performing any high temperature processes, significant yield losses may occur during conventional process strategies due to significant damage of the interlayer dielectric material 121 prior to and upon replacing the placeholder material with at least a metal-containing electrode material.
In view of the situation described above the present disclosure relates to manufacturing techniques and semiconductor devices in which a replacement gate approach may be applied while avoiding or at least reducing the effects of one or more of the problems identified above.